Tensioning devices, such as hydraulic tensioners, are used as a control device for a power transmission chain, or any similar power transmission devices, as the chain travels between a plurality of sprockets. Generally, it is important to impart and maintain a certain degree of tension to the chain to prevent noises, slippage, or the unmeshing of teeth (in cases of a toothed belt). Prevention of such slippage is especially important in the case of a chain driven camshaft in an internal combustion engine because jumping of teeth will throw off the camshaft timing by several degrees, possibly rendering the engine inoperative or causing damage. However, in the harsh environment in which an internal combustion engine operates, belt tension can vary between excessively high or low levels as a result of the wide variations in temperature and differences between the coefficients of linear expansion among the various parts of the engine, including the chain and the tensioner. Moreover, wear to the chain components, during prolonged use, can result in a decrease in the tension of the chain. Thus it is also necessary to provide some measures to remove excessive tensioning forces on the tight side of the chain and to insure the necessary tension forces on the slack side of the chain. In addition, cam shaft and crank shaft induced torsional vibrations cause belt tension to vary considerably. This tension variation results in chain elongation, determined by belt stiffness.
One example of a device used to control tension in a wrapped power transmission device is described in Kimura et al., U.S. Pat. No. 4,708,696. Kimura et al. discloses a hydraulic ball-type check valve tensioner having a plunger slidably fitted into a chamber and biased by a spring in a protruding direction. An extending rod imparts tension according to the degree of slackening of the chain. A clearance, formed between the ball and seat of a check valve, permits the free flow of fluid therethrough. Therefore, the hydraulic pressure from an external source, such as an oil pump or the like, flows into a chamber through passages formed in the housing, advancing the plunger easily by the combined efforts of the hydraulic pressure and the spring force.
On the other hand, when the plunger tends to move in a reverse direction, the ball is tightly contacted with the ball seat to restrict outflow of fluid from the chamber. Only a small clearance between the plunger and the housing wall permits some fluid to escape thereby allowing the plunger to retract. In such a fashion the tensioner achieves a so-called no-return function, i.e., movements are easy in one direction but difficult in the reverse direction.
However, the above-described tensioner construction fails to always maintain a predetermined tension, such as when an engine is idling or at rest with low or no oil pressure. Unless appropriate oil pressure is applied to the chamber or it is filled with sufficient oil, the plunger becomes easily movable in both directions and loses the no-return function, producing vibrations and noises due to the failure in maintaining a predetermined degree of tension in the chain. Moreover, the response time of a ball type check valve to open or close to or from a full flow condition is somewhat limited. Additionally, the use of a ball check valve requires the use of a complicated, costly structure which is difficult to assemble. In contrast, a spring check valve, such as that disclosed in Paul Jr. U.S. Pat. No. 3,896,834, offers several advantages. Specifically, such a check valve is of simple construction and features a more rapid response time than a ball type check valve to open or close to or from a full flow condition.
The provision of a ratchet, formed on the outer surface of the plunger, as disclosed in Suzuki U.S. Pat. No. 4,874,352, provides a mechanical no-return mechanism which ensures the plunger will remain extended outwardly even upon low oil pressure. Such a design, however, because it prevents the plunger from having a smooth cylindrical outer surface, causes problems with sealing the plunger cavity and maintaining pressure to the plunger.
Providing a ratchet internal to the plunger is therefore desirable. Renold Great Britain Patent 1,018,211 discloses a helical channel for use in such a tensioner. The helical channel engages with a cantilevered pin on the plunger. Because the pin is cantilevered however, it is susceptible to bending. In addition, because only one side of the internal ratchet is engaged with the plunger, the distribution of forces between the internal ratchet and plunger is asymmetrical, i.e. only one pin extending from one side of the interior wall of the plunger engages into the helical channel. Jamming of the internal ratchet in the plunger is therefore more likely.
Accordingly, it is an object of the present invention to provide a tensioner for chain, belt or similar wrapped power transmission devices which can maintain a substantially constant tensioning force.
It is another object of the present invention to provide a hydraulic tensioner which can provide a degree of tension to a chain or the like when the fluid pressure to the tensioner is low or zero.
It is a further object of the present invention to provide a hydraulic tensioner having an improved internal ratchet system to mechanically maintain the tension of a chain or the like.
It is also a further object of the present invention to provide a hydraulic tensioner which can be reliably sealed to thereby maintain fluid pressure in the tensioner.
It is also a further object of the present invention to provide a hydraulic tensioner which has a reliable design to ensure the internal ratchet and plunger operate smoothly and efficiently.
It is also a further object of the present invention to provide a hydraulic tensioner which has a check valve configured to offer a high frequency response and rapid opening or closing to or from a full flow condition.